1. Field of the Invention
This invention relates to electrochemical oxygen sensors, and particular to the adaptation of commercially available metal-air batteries that are normally used to supply power for hearing aids and similar devices for use as oxygen sensors of high precision and repeatability. The use of a commercially available battery already being manufactured in high volumes, such as zinc-air hearing aid batteries, will provide low cost, disposable oxygen sensors. The combination of low cost and high performance will make them suitable for a number of applications including confined space monitoring for worker safety, medical applications relating to anaesthesiology, respiratory therapy and the like, and the monitoring of industrial processes where the oxygen concentration level is critical.
2. Description of Related Art
Prior art electrochemical oxygen sensors have been based on custom designed electrode/electrolyte systems housed in custom fabricated housings. This approach has led to a number of successful oxygen sensors that are costly to produce, examples of which are disclosed in U.S. Pat. Nos. 2,913,386, 3,767,552 and 4,132,616. Devices manufactured according to these patents tend to be expensive because they are custom designed for the purpose of detecting oxygen, and are produced in small manufacturing volumes relative to common batteries. Zinc-air batteries, on the other hand, are metal-air batteries that are manufactured in large quantities and at a very low per-unit cost.
Prior art disposable oxygen sensors have employed a polarizable cathode of carbons and metals, and metallic anodes that are generally nonpolarizable at the relatively low current densities found in these sensors. In all of these sensors the cathode is maintained at a potential that is sufficiently negative relative to the electrolyte to promote the reduction of the oxygen on the cathode surface. This potential, or bias, for the cathode is obtained in several ways depending on the type of materials used for the cathode and anode, and depending on the choice of electrolyte between cathode and anode.
As disclosed in U.S. Pat. No. 2,913,386, the classic Clark oxygen sensor uses a platinum cathode and a silver-silver chloride anode, with an aqueous sodium chloride electrolyte; the cathode and anode are biased externally with a constant potential of 0.8 volts maintained between sensor terminals (cathode made more negative than anode). Without the external voltage source the cell does not respond to oxygen.
A similar sensor described in U.S. Pat. No. 3,000,805 uses the same anode and cathode materials, but the electrolyte is either aqueous potassium chloride or potassium hydroxide, the latter being recommended. The potential of the cathode is held constant at 0.9 volts negative with respect to the anode. In both of these disclosures the potential between electrodes is held constant with a battery and variable resistor.
The practice of operating oxygen sensors at fixed potential, with a potentiostat or other suitable electric circuit has been widely adopted (I. Fatt, "Polarographic Oxygen Sensors", Robert E. Krieger Publ. Co., Malabar, Fla., 1982). Other oxygen sensors, referred to as fuel cell sensors, supply their own power and bias to the cathode by the consumption of a fuel at the anode. The anode, via the external electrical circuit is a source of electrons for the cathode. In some cases the anode can be a catalytic electrode that consumes a gaseous fuel such as hydrogen, but in many cases the anode is a metal that corrodes in the presence of the reaction product of the cathode. These types of fuel cell sensors are known as galvanic oxygen sensors. Examples of such sensors are disclosed in U.S. Pat. Nos. 3,767,552 and 4,132,616 which use lead (Pb) as the consumable anode with a caustic electrolyte.
U.S. Pat. No. 2,805,191 cites the use of cadmium, lead, antimony arsenic, and copper as possible anode materials with various electrolytes. Sensors using these metals as anodes are unique in being able to operate with only a simple load resistor to complete the circuit between the anode and the cathode.
In the prior art, zinc has been specifically excluded as a possible galvanic oxygen sensor anode, with any electrolyte, by both U.S. Pat. Nos. 2,805,191 and 4,132,616, because zinc can cause hydrogen evolution at the cathode in the absence of oxygen. Tin has also been avoided with caustic electrolytes due to the risk of hydrogen evolution. Zinc however offers many advantages in this application compared to alternative metals such as lead, including availability at relatively low cost, high energy density and relatively environmentally benign properties.
The problems of hydrogen evolution associated with the use of zinc as the oxygen sensor anode do not occur when zinc is used as the anode in a power generating metal-air battery at normal atmospheric oxygen concentrations. In a typical battery application most of the zinc potential appears across the battery load, and the air electrode is only moderately polarized into the cathodic region, thus avoiding hydrogen evolution. However, in a typical galvanic oxygen sensor, the percentage of the zinc potential applied to the air electrode always approaches 100 percent as the oxygen concentration goes to zero and/or the load resistor is reduced to improve sensor response time.
This high potential across the cathode under low oxygen conditions results in the evolution of hydrogen at the cathode, producing as a consequence an erroneous signal with a high baseline and the risk of cell leakage due to the buildup of hydrogen gas within the sensor. The smaller the load resistor in such an oxygen sensor, the sooner hydrogen evolution begins as the oxygen level drops toward zero before the rate of hydrogen evolution becomes significant. It is not necessary for the oxygen level to be reduced all the way to zero.
The high surface area electrodes commonly used in zinc-air cells behave as though the electrodes have a large capacitance, defined as C=dQ/dE, where dQ is the charge passed for a small change in potential dE. The combination of this capacitance with the load resistance and cell resistance results in the response time of the sensor to changes in oxygen concentration being determined by the electrical time constant(s) of the cell and external circuit. The use of large load resistors to reduce the magnitude of the hydrogen evolution process results in long sensor response times. For practical implementation, the use of a zinc-air cell with a simple load resistor produces an unacceptable compromise between excessive response time and lower limit of measurable oxygen range prior to the onset of hydrogen evolution and associated unacceptable baseline signals.
U.S. Pat. No. 4,132,616 indicates that metals such as zinc and tin may be used if an external bias circuit is used to keep the potential of the cathode away from the hydrogen evolution region. The bias potential needs to be held at a value where oxygen reduction occurs under diffusion control and yet hydrogen evolution does not yet occur, as with the other so called controlled potential oxygen sensors discussed above.
One example of an oxygen sensor based on a zinc-air cell is disclosed in U.S. patent application Ser. No. 08/620,944, filed Mar. 22, 1996, in which a controlled potential of -0.6V was applied to the cell. The cell used in this example was a zinc-air battery design which had been internally modified to make it suitable for use as an oxygen sensor.
The evolution of hydrogen in oxygen sensors using zinc metal anodes in some cases may be used to provide the sensing mechanism, such as the Jacobson cell in which the oxygen depolarizes the hydrogen formed at the outer surface of a porous hollow carbon rod cathode (Jacobson, M. G.; Analytical Chemistry, (1953), 25, 585). A similar hydrogen depolarization mechanism is described in U.S. Pat. No. 4,664,119 in which any metal more electronegative than hydrogen can be used for the anode.
Another aspect of state of the art oxygen sensors as described by U.S. Pat. Nos. 3,767,552 and 4,132,616 is the use of a diffusion barrier between the oxygen atmosphere being measured and the oxygen electrode inside the sensor. The oxygen sensor signal magnitude is then proportional to the rate of diffusion through the barrier, provided that all the oxygen reaching the electrode is reduced. In order to operate in this mode, the oxygen cathode must have sufficient reserve activity that the cathode reaction is controlled by diffusion. Variations in cathode activity due to manufacturing variations and ambient temperature changes are then either small or undetectable in sensor output current response to oxygen concentration. In addition to controlling the signal from the oxygen sensor, the diffusion barrier reduces the rate at which sensor contaminants enter the sensor and also reduces the rate at which water from the electrolyte is exchanged with the atmosphere outside the sensor.
The diffusion barrier can be a solid polymer membrane, as in the case of U.S. Pat. No. 3,767,552, a small orifice or capillary as in the case of U.S. Pat. No. 4,132,616, or a porous body as in the case of U.S. Pat. No. 4,446,000.
The design of the diffusion barrier is also a useful means for optimizing the sensor for a particular concentration or range of concentrations of oxygen to be monitored. For instance, when measuring oxygen concentrations near 100%, a more restrictive diffusion barrier may be desirable to assure sensor responsivity. At lower concentrations a less restrictive barrier may be desirable to provide a larger signal.
A metal-air battery employs diffusion limiting membranes similar to the oxygen sensors described above, but the intended rate of diffusion is much higher than needed to detect oxygen. While the higher oxygen consumption rate is essential for the battery to supply high output current, it is a disadvantage in the case of the oxygen sensor where the higher cell current unnecessarily shortens sensor life. A more restrictive opening to the atmosphere also better protects the electrodes and electrolyte against damage from humidity changes and contaminants outside the sensor and provides for a more stable output signal as environmental conditions change and as the cell ages.